Add django 2.1 to tested envs

tox.ini

@@ -2,6 +2,7 @@
 envlist =
     py{27,34,35,36,37}-django111
     py{34,35,36,37}-django20
+    py{35,36,37}-django21
     lint
 
 [testenv]
@@ -10,7 +11,8 @@ extras = test
 pip_pre = true
 deps =
     django111: django>=1.11,<2.0
-    django20: django>=2.0
+    django20: django>=2.0,<2.1
+    django21: django>=2.1
     appdirs==1.4.3
     py27: mock==2.0.0